1. Field of the Invention
The present invention relates to an improved process for the preparation of products having improved flow properties, in particular to a process for the preparation of agglomerated tetrafluoroethylene (TFE) polymer molding powders.
2. Description of the Prior Art
Granular polymers of high molecular weight are generally manufactured by polymerization of tetrafluoroethylene, either alone or in the presence of a comonomer in an aqueous medium.
In order to improve the physical properties of articles made from such granular polymers, the polymer is disintegrated after polymerization. The best physical properties are obtained when the polymer has been disintegrated to a very small size--e.g., a polymer conforming to ASTM D4894 Type 2 specification.
However, such finely disintegrated polymer powders have very poor powder-flow properties and a low bulk density. These two disadvantages render the polymer unsuitable for use in automatic molding machines and so it is desirable to improve the bulk density and powder-flow properties.
The powder flow and bulk density of the finely disintegrated powders may be improved by subjecting the finely divided polymer powder to an agglomeration process. The aim of such a process is to cause the finely divided particles to adhere together to form agglomerates which exhibit good powder-flow properties and which also have a low compaction ratio but which can be broken down upon molding of the polymer powder so that the good mechanical properties obtained by the use of finely disintegrated material are retained.
Various methods have been proposed for effecting such agglomeration. One method of agglomeration is described in U.S. Pat. No. 3,597,405 in which the polymer powder is wetted with a minor amount of a liquid having a low surface tension and the wetted powder is formed into agglomerates by, for example, a tumbling operation. After formation of the agglomerates, the liquid is removed by heating. However, the liquids used in that process are generally volatile organic materials and the heating operation gives rise to flammability or toxicity problems. In addition it would be desirable to use cheaper liquids than these organic liquids.
U.S. Pat. No. 3,682,859 describes a similar process in which the tetrafluoroethylene polymer is wetted with an organic liquid.
U.S. Pat. No. 3,882,217 describes an agglomeration process in which the tetrafluoroethylene polymer product is wetted with a small amount of a solution of a volatile non-ionic surfactant in water prior to agglomeration. However, the surfactants decompose at the temperature used to dry the agglomerates and the decomposition products adversely affect the mechanical properties and color of products prepared from the agglomerates.
U.S. Pat. No. 3,983,200 describes a similar process in which tetrafluoroethylene polymer is first wetted with an aqueous solution containing a protective colloid.